In the past, it has been possible to produce a corrugated tube or ribbed tube formed of thermoplastics material by extrusion of a tubular parison into a travelling mold tunnel and molding the external surface of the tube on a mold surface of the mold tunnel. The mold tunnels themselves may be provided in a variety of different manners. For example, a mold tunnel may be formed of two co-operating chains of mold block parts which close together along a forward run to form the mold tunnel in any manner, For example, mold block parts may be hinged together to close so that adjacent mold blocks form a mold tunnel and to open at a downstream end of the mold tunnel to release tube from therein, or may be separate and close to form the tunnel by means of co-operating parallel runs of blocks. A conventional apparatus for provision of a mold tunnel for a single walled corrugated tube is described and claimed in U.S. Pat. No. 3,981,663 issued to Gerd P. H. Lupke on Sep. 21, 1976. A travelling mold tunnel utilizing mold block parts which are hinged together and which open at a downstream end of a molding run to release tube form therein is described and claimed in PCT Patent Application CA90/00327 which was published on May 16, 1991 under the publication number WO91/06419 (now U.S. Pat. No. 5,372,774). One particular apparatus for the production of ribbed pipe is described and claimed in U.S. Pat. No. 4,712,993 issued to Manfred A. A. Lupke on Dec. 15, 1987. The patents and patent application referred to are mentioned by way of example only. Numerous publications are available concerning modifications of previously known apparatus.
It has generally been believed that the use of a travelling mold tunnel is not possible for making smooth walled tube. In all cases of the use of a travelling mold tunnel for making corrugated or ribbed tube, the mold tunnel is provided with an annularly or helically grooved mold surface to form annular or helical grooved corrugations or ribs in the resulting product. Such grooves in the wall of the mold tunnel are filled, during molding, with a layer or mass of thermoplastic material. As the mold tunnel advances, such layer or mass in a groove of the mold wall acts to provide traction acting to help drag the parison or partially molded tube forward along the length of the mold tunnel.
The presence of such grooves in the wall of a travelling mold tunnel are necessary for molding purposes when corrugated or ribbed tube is being formed and their use as transport aids may either have been unappreciated or rarely or never discussed. It has, however, been appreciated that it is not possible to form smooth walled tube by means of a conventional travelling mold tunnel. This is, in fact, due to the lack of traction for the tube during the molding process. Thus, a travelling mold tunnel which has an entirely smooth wall will have no traction means to help drag the parison or material being molded with it as it advances. When ribbed tube is formed, it has been necessary to provide spaces between the ribs which are not so great that the thermoplastics material tends to slip in the mold in that region. If the ribs are too widely spaced apart, the flow of plastic will not be even and slippage will occur between the unset thermoplastics material and the mold tunnel. Resultant ugly, weakened, roughened patches or wavey lines present on the surface indicate irregular transport of the material due to slippage in the mold.
In some cases, when ribbed tube is being provided, it has been the practice to make such tube with main ribs which are spaced apart by as much as one inch or more, the main ribs upstanding from the surface of the tube by as much as an inch or more. The distance between the main ribs may be provided with smaller riblets in the form of annular bands which upstand from the surface of the tube by a small amount. The provision of these annular riblets has been primarily for the purpose of providing seats for O-rings or for strength purposes of the tube or for reasons of appearance. Generally, one, two or three such bands may be provided in the interval between main ribs; although more than three such bands may be present. However, it is believed that no appreciable number has been used.
Corrugated tube made by conventional methods in a travelling mold tunnel may be provided with belled or otherwise shaped sections by the provision of special mold blocks. Such sections have traditionally had smooth walls but are usually short in length and have not caused any major difficulties in production. Such short sections of plain walled belled tube are provided, for example, for sockets for joining two lengths of tube together. The production of similar belled lengths of ribbed tube has caused considerably more difficulty. Ribbed tube is formed in a travelling mold tunnel by injection molding of a parison of thermoplastics material between an inner forming plug and the mold tunnel molding surface. This injection molding process in a travelling mold is much more prone to shearing of the thermoplastic parison in molding due to difficulties of precise control in transporting the parison with the mold.
Smooth walled tubing having no ribs or corrugations on either its inner or outer surfaces has traditionally been made using different elaborate apparatus. A parison of thermoplastic material may be either extruded over a core to form the inner surface or into a vacuum mold. Beyond the initial forming stage, various cooling jackets and other equipment is necessary. The production line for smooth walled tubing tends to be longer than that which is possible using a travelling mold tunnel. Travelling mold tunnels, however, have not been found suitable for the production of smooth walled tubing due to the difficulties in transporting the parison without shear between an inner forming plug and an outer mold tunnel.
If it had been possible to use a travelling mold tunnel for the production of smooth walled tubing, it is believed the process might well have been an injection molding process as used for ribbed tubing rather than the less critical blow molding process used for corrugated tube. It is, as previously commented, just such an injection molding process which causes the most difficulties with shear in the transport of the parison in the travelling mold tunnel. Hypothetical future developments apart, it is generally believed that the molding of smooth walled .tube in a travelling mold tunnel is not practicable.
Consideration of the difficulties involved in the production of ribbed tubing in a travelling mold tunnel in which the interval between main ribs is sufficiently long that one rib is fully formed before the formation of another is started, has been considered in U.S. Pat. No. 4,900,503, but the thrust of that disclosure is towards the proper filling of the rib cavities. There is no consideration of molding smooth walled tube. In that patent of Wilhelm Hegler, there is described and claimed a method and apparatus for the production of ribbed pipe in which the intervals between the ribs are provided with saw tooth annular snags due to the provision of saw tooth shaped indentations in the mold tunnel for the purpose of transportation of the parison.